1. Field of the Invention
This invention relates to an electric vehicle control system, and more particularly to an electric vehicle control system which controls a permanent magnet synchronous motor for driving an electric vehicle.
2. Description of the Related Art
In order to run an AC motor driven electric vehicle controlled by variable voltage variable frequency inverters ("inverters") smoothly, the electric vehicle control system should be designed in such a way that even if, by chance, an inverter should malfunction, it is possible to continue driving the electric vehicle by opening the malfunctioning part.
In general, a 3-phase induction motor is typically used as an electric vehicle drive motor. However, recently an electric vehicle has been developed which is driven by permanent magnet synchronous motors ("PM motors") supplied respectively with 3-phase AC power from inverters. PM motors can be broadly divided into two types. The first type are PM motors having a surface magnet structure in which permanent magnets are attached to the rotor surface of the motor. The second type are PM motor having a buried magnet structure in which permanent magnets are buried inside the rotor. PM motors are superior in maintainability, controllability and ability to withstand the environment, and are capable of being operated with high efficiency and high power factor as compared to other types of motors. Therefore, PM motors possess desirable characteristics and features as electric vehicle drive motors.
FIG. 2 is a schematic diagram of a prior art electric vehicle control system which controls one PM motor 5 with one inverter 3. DC power collected from an overhead power line (not illustrated) via a pantograph 1 passes through a line breaker 2 which switches the current ON and OFF. The DC power is, then, converted to a variable voltage variable frequency AC power by the inverter 3, and is supplied to the PM motor 5. A control device 4 receives information P from sensors (not illustrated) mounted in inverter 3 and information R, such as a speed of revolution and an angle of rotation of PM motor 5. Then, based on information R, control device 4 calculates an inverter frequency and a motor voltage, and outputs these as a control signal C. Inverter 3 is controlled based on this control signal C. Inverter 3 is composed of self-turn-off semiconductor devices 3a-3f ("semiconductor devices"), such as GTO thyristors or IGBTs, which are capable of being controlled by control signal C from control device 4 so as to be placed conductive or non-conductive states with predetermined timing. Diodes 31 are respectively connected in an antiparallel fashion with the self-turn-off semiconductor devices 3a-3f.
FIG. 3 is a drawing showing the operation in the prior art electric vehicle control system shown in FIG. 2 when semiconductor device 3a of semiconductor devices 3a-3f malfunctions so as to be in a state of constant conduction. When semiconductor device 3a is in a conduction malfunction state, inverter 3 cannot supply 3-phase AC power to PM motor 5. When control device 4 detects the conduction malfunction of semiconductor device 3a via information P from sensors mounted in inverter 3, control device 4 outputs an opening instruction al to line breaker 2. Therefore, the operation of inverter 3 is stopped by placing the line breaker 2 into an open state. In this case, generally, the electric vehicle continues to be operated with 3-phase AC power supplied to other PM motors 5 from other fault-free electric vehicle control systems, respectively. However, when the electric vehicle continues to be operated, the rotor of PM motor 5 connected to malfunctioning inverter 3 continues to rotate. Since PM motor 5 is composed of permanent magnets, a magnetic flux is generated inside PM motor 5, even when AC power is not supplied from inverter 3, and PM motor 5 operates as a generator. At this time, if all of semiconductor devices 3a-3f are in a fault-free state, the system is designed such that the current does not continue to flow from PM motor 5. However, when semiconductor devices 3a has a conduction malfunction, short-circuit currents flow between the phases of PM motor 5 via the routes shown by the arrows in FIG. 3. Therefore, a problem arises that, if the electric vehicle continues to be operated in this state, it may result in PM motor 5 burning out through overcurrent and overheating due to the short-circuit currents.
In a prior art electric vehicle control system such as described above, when any semiconductor device of an inverter has a conduction malfunction, the power supply from the overhead supply line side to the malfimctioning inverter is cut off by the line breaker and the electric vehicle continues to be operated by other fault-free electric vehicle control systems.
However, short-circuit currents continue flow in the PM motor via the malfunctioning inverter. As a result, there arises the problem of burn-out of the PM motor. Therefore, the problem arises that the operation of the electric vehicle itself cannot be continued, and thus commercial operation thereof will be hindered.